Interference reduction for radio signal receivers



July 13, 1948. w. B. SMITH 2,445,157

INTERFERENCE REDUCTION FOR RADIO SIGNAL RECEIVERS Filed June 1, 1944 (m To Healers Patented July 13, 1948 INTERFERENCE REDUCTION FoR RADIO SIGNAL RECEIVERS Wilbert Brockhouse Smith, Ottawa, Ontario, Canada Application June 1, 1944, Serial No. 538,266 In Canada May 26, 1944 8 Claims. (Cl. 25020) This invention relates to radio systems and more particularly to a method and means for improving the signal-to-noise ratio of the output from a radio receiver.

It is an object of the invention to provide a radio noise filter of the regenerative type which may be applied to various forms of receivers, which satisfactorily increases the signal-to-noise ratio under conditions which have hitherto rendered difiicult the production of intelligible signals, which is subject to simple adjustment when placed in service, and which requires little or no adjustment during its operative life.

It is common practice to augment the wanted components in an incoming signal, which includes unwanted components such as noise, by regenerative action, and such regeneration may be accomplished in many ways.

While the present invention includes the use of regenerative action to better the signal-tonoise ratio, it particularly contemplates the employment of such action in a novel combination and in association with novel features.

The invention broadly comprises (1) the regeneration of a normal incoming signal plus noise initially to increase the signal-to-noise ratio, and (2) the balancing of the normal incoming signal plus noise against the regenerated signal plus noise substantially to balance out or eliminate the unwanted components in the regenerated signal and leave the amplified wanted signal. Efiicient functioning of the invention depends upon the attainment of maximum regenerative action. This is attained when the voltage of the incoming signal plus noise, as delivered to the vacuum tube performing the regeneration, is comparable to but less than the minimum voltage required to maintain self-excited oscillation. Thus, it is desirable to maintain the incoming signal at a relatively constant value. Therefore, the invention also contemplates the use of an automatic volume control adapted to receive the signal from its source and deliver it at substantially constant value for the regenerative action referred to above.

The invention will be described with particular reference to the accompanying drawing, in which Figure 1 is a circuit diagram indicating the chief constituents of a filter installation in accordance with the present invention, and

Figure 2 is a circuit diagram of a typical filter installation in a radio receiver.

While any suitable automatic volume control means may be employed for the purposes of the invention, that hereinafter described is particu- 2 larly satisfactory for the purpose and is believed to possess novel characteristics.

Referring to Figure 1, the volume control portion of the circuit comprises a limiting amplifier including a rectifying vacuum tube I, such as a type 61-16, and a vacuum tube 2 of the pentode type having remote cut-off characteristic. The latter tube may be, for instance, a type 6K7, although other types, such as (D6 and 607, may be employed. The input signal is applied through transformer 4 to the screen grid 3 of tube 2. A portion of the input signal is rectified by the tube l in circuit 5, and is applied negatively to the control grid t; of tube 2; The circuit 5 includes the condensers l and 8 and resistances 9, i9 and H to form a resistance-capacity network which is provided in the rectifying circuit in order to introduce an appreciable time lag between the application of the rectified voltage to the control grid 6 and its dissipation. The resistors 9 and II] also serve to. reduce the voltage avail able-from the rectifying circuit to an appropriate value for actuatingthe control grid 6 of tube 2. It is a characteristic of tube 2, when used in the manner described, that the amplification factor between the screen grid and plate circuit is reduced as the negative bias on the control grid is increased. The proportion of rectified signal voltage which is applied to the control grid 6 is so chosen that the decrease in amplification of tube 2 approximately offsets the increase in the incoming signal, with the net result that the output delivered by the plate circuit of tube 2 is approximately constant over a fairly wide range of input signal variation.

'The regenerative filter itself consists of two portions, a filter proper and a compensating or balancing circuit.

The filter proper comprises a transformer l2, the primary winding is of which is connected to the plate M of tube 2, and the secondary winding I 5 of which is coupled to the grid Hi and connected to the plate ll of a vacuum tube 3 of the detector amplifier trioole type, such as a 6J5, in such a manner as to form a self-excited oscillator which, by means of regeneration control means 34, is operated just below the point of oscillation. The secondary i5 is tuned, by means of a shunt condenser l9, to-a center or characteristic frequency. The center frequency which is equal to, or nearly equal to, the frequency of the wanted component of the incoming signal, which must be essentially of a single frequency such as, for instance, a keyed continuous wave telegraph signal.

The balancing circuit comprises a transformer 20, which is substantially identical with transformer l2. The primary winding 2| of transformer 2|] is connected in series with the primary winding I3 of transformer I2, and the secondary winding 22 of transformer 20 is connected inphase opposition with the secondary winding I5 of transformer I2 so that the voltages induced in the secondaries of the two transformers oppose. The secondary 22 is also tuned to the center or characteristic frequency by means of a shunt condenser 23. A potentiometer or potential divider 24 is also included in the balancing circuit.

In operation, the incoming signal, consisting of the wanted component, which must be close in frequency to the center or characteristic frequency, to which the transformers I2 and 20 are tuned, together with the unwanted components such as noise, is injected into the transformers. In the filter portion proper, these various components undergo a certain amount of amplification which in the case of the wanted component is greatly augmented by the regenerative action of the transformer IZ-tube I8 combination. Since there is no amplification taking place in the balancing circuit, the only effect on the ratio of wanted signal to unwanted background noise is that due to the resonant characteristics of the transformer 20, which are identical to the eifects taking place in the equivalent circuit in the filter portion. Therefore, by means of the potentiometer 24, shunted across the balancing circuit, a voltage equal and opposite to the normal unamplified signal plus noise is balanced against the total signal available in the filter portion and as a result the unwanted components are largely balanced out, leaving the amplified wanted component. The resultant signal voltage appearing across the points 25 is then applied to the grid of an ordinary type of amplifier.

The circuit diagram shown in Figure 2 is illustrative of a practical form of the invention which may be associated, forinstance, with a tape recording device for automatically recording radio signals. As is well known, such automatic tape recorders operate only in response to a substantially clear signal with a minimum background of noise. It will be observed that the circuit includes the various elements of the circuit illustrated in Figure 1, such elements being'indicated by like reference characters. The circuit, as shown, includes a suitable type of power supply comprising the transformer 26, an ordinary type 8D rectifier tube 28, and filter section 29. The circuit also includes an amplifier stage comprising an ordinary amplifier tube 30, such as a type 6J5, to the grid 3| of which the filtered'signal is applied. The tube 3!] is connected to an output transformer 32. A switch 33, withaccompanying connections, may be included to provide aimeans for placing the unit in or out of operation.

It has been found that a filter device, such as described, may be utilized to deliver a relatively constant level signal to a tape recorder whereby the operation of the latter proceeds satisfactorily and without interruption during substantially. all conditions of reception.

It is to be understood, however, that the invention may be applied to various types of radio receivers, including the domestic type of receiver of the superheterodyne type, when the center or characteristic frequency is made equal to the intermediate frequency, and the filteriis inserted in the intermediate frequency amplifier of the receiver.

It is a feature of the device described that substantially the only adjustments required therein consist in the setting of the regeneration and balancing controls made when the device is placed in service. When once set, these controls need not be adjusted again unless the device is retubed with tubes having widely different characteristics.

It will be understood that various changes in component parts and circuit arrangements may be made without departing from the spirit and scope of the invention as defined in the appended claims.

I claim:

1. Method of improving the signal-to-noise ratio of the output from a radio receiver which comprises subjecting the normal input signal plus noise to a regenerative action to amplify the wanted signal in said signal plus noise, andthen subjecting said amplified signal plus unemplified noise to a balancing action wherein a voltage equal and opposite to that of the normal unamplified signal plus noise is balanced against the amplified signal plus unamplified noise substantially to eliminate the unamplified noise and leave said amplified signal.

2. Method of improving the signal-to-noise ratio of the output from a radio receiver which comprises controlling the volume of the normal input signal plus noise to maintainv it at a substantially constant level, subjecting said volume controlled signal plus noise to a regenerative action to amplify the wanted signal therein, subjecting the amplified signal plus unamplified noise to a balancing action wherein a voltage equal and opposite to that of the normal unamplified signal plus noise is balanced against the amplified signal plus unamplified noise substantially to eliminate the unamplified noise and leave said amplified signal, and finally subjecting said amplified signal to normal output amplification.

3. A noise eliminating system for radio receivers comprising a regenerative filter section and a balancing section connected in series in phase opposition, means for injecting a normal input signal plus noise into both said sections, said filter section having means for amplifying substantially only the wanted components of said signal plus noise, means for applying said amplified wanted components plus noise to said balancing section and said balancing section having means for substantially balancing out the unamplified components of said signal plus noise.

4. A noise eliminating system for radio receivers comprising a regenerative filter section and a balancing section connected in series in phase opposition, each tuned to a characteristic frequency, means for injecting a normal input signal plus noise into both said sections, said filter section having means for amplifying substantially only the components of said signal plus noise of substantially the same frequency as said characteristic frequency, said components comprising the wanted components of said signal plus noise, means for applying said amplified tone components plus noise to said balancing section said balancing section having means for balancing against the voltage of said amplified components plus noise a voltage equal and opposite to that of the normalunamplified signal plus noise thereby substantially to eliminate the noise.

5. In a radio receiver having volume control means for maintaining the volume of the normal input signal plus noise of said receiver at a substantially constant level, means for improving the signal-to-noise ratio of said signal plus noise comprising a filter section and a balancing section each tuned to a characteristic frequency and connected in series in phase opposition, means for injecting said signal plus noise at constant level into both said sections, said filter section having regeneration means for amplifying substantially only the components of said signal plus noise of substantially the same frequency as said characteristic frequency, said components comprising the wanted components of said signal plus noise, means for applying said amplified tone components plus noise to said balancing section, said balancing section having means for balancing against the voltage of said amplified components plus noise a voltage equal and opposite to that of the normal unamplified signal plus noise thereby substantially to balance out said noise.

6. In a radio receiver, means for improving the signal-to-noise ratio of the normal signal plus noise input of said receiver comprising a filter section and a balancing section, said filter section comprising regeneration means including a transformer and a vacuum tube, said balancing section also including a transformer, means connecting the primary windings of said transformers in series and to said signal plus noise input, means connecting the secondary windings of said transformers in phase opposition, means for tuning each of said secondary windings to a characteristic frequency, and a potential divider across the tuned secondary circuit of said second transformer.

7. In a radio receiver, means for improving the signal-to-noise ratio of the normal signal plus noise input of said receiver comprising a filter section and a balancing section, said filter section comprising regeneration means including a transformer and a vacuum tube, said balancing section also including a transformer, means connecting the primary windings of said transformers in series and to said signal plus noise input, means connecting the secondary windings of said transformers in phase opposition, the secondary winding of said first transformer having a circuit connected to said vacuum tube, the secondary winding of said second transformer also having a circuit, a shunt condenser in each of said secondary winding circuits for tuning said secondary windings to a characteristic frequency, and a potential divider across said second secondary winding circuit.

8. In a radio receiver having volume control means including an amplifier tube for maintaining the volume of the normal signal plus noise input of the receiver at a substantially constant level, said amplifier tube having a plate, means for improving the signal-to-noise ratio of said input comprising a filter section and a balancing section, said filter section comprising regeneration means including a transformer and a vacuum tube, said balancing section also including a transformer, means connecting the primary windings of said transformers in series and to the plate of said amplifier tube, means connecting the secondary windings of said transformers in phase opposition, the secondary winding of said first transformer having a circuit connected to said vacuum tube, the secondary winding of said second transformer also having a circuit, means for tuning each of said secondary windlugs to a characteristic frequency, and a potential divider across the secondary winding circuit of the second transformer,

WILBERT BROCKHOUSE SMITH.

7 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,476,691 Cohen et al Dec. 11, 1923 1,589,979 McCaa June 22, 1926 2,138,123 Sargent Nov. 29, 1938 2,273,673 Van Loon Feb. 17, 1942 2,298,297 Jacob Oct. 13, 1942 2,318,075 Hollingsworth May 4, 1943 FOREIGN PATENTS Number Country Date 303,608 Great Britain Jan. 10, 1929 308,312 Great Britain May 1, 1930 792,772 France Jan. 1936 471.065 Great Britain Aug. 24, 1937 

